1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
There are some power conversion circuits which accomplish higher efficiencies by implementing a mechanism that accomplishes switching at zero voltage. Power loss in a switch is the product of the voltage applied across the switch and the current flowing through the switch. In a switching power converter, when the switch is in the on state, the voltage across the switch is zero, so the power loss is zero. When the switch is in the off state, the power loss is zero, because the current through the switch is zero. During the transition from on to off, and vice versa, power losses can occur, if there is no mechanism to switch at zero voltage or zero current. During the switching transitions, energy losses will occur if there is simultaneously (1) non-zero voltage applied across the switch and (2) non-zero current flowing through the switch. The energy lost in each switching transition is equal to the time integral of the product of switch voltage and switch current. The power losses associated with the switching transitions will be the product of the energy lost per transition and the switching frequency. The power losses that occur because of these transitions are referred to as switching losses by those people who are skilled in the art of switching power converter design. In zero voltage switching converters the zero voltage turn off transition is accomplished by turning off a switch in parallel with a capacitor and a diode when the capacitor""s voltage is zero. Often the capacitor and the diode are intrinsic parts of the switch. The capacitor maintains the applied voltage at zero across the switch as the current through the switch falls to zero. In the zero voltage transition the current in the switch is transferred to the parallel capacitor as the switch turns off.
The zero voltage turn on transition is accomplished by discharging the parallel capacitor using the energy stored in a magnetic circuit element, such as an inductor or transformer, and turning on the switch after the parallel diode has begun to conduct. During the turn on transition the voltage across the switch is held at zero, clamped by the parallel diode. The various zero voltage switching (ZVS) techniques differ in the control and modulation schemes used to accomplish regulation, in the energy storage mechanisms used to accomplish the zero voltage turn on transition, and in a few cases on some unique switch timing mechanisms.
There are a few examples of zero voltage switching cells and power converters that contain zero voltage switching cells that have been patented. Examples include U.S. Pat. No. 6,198,260 and U.S. Pat. No. 6,259,235.
In most cases the zero voltage switching cells enable the switching frequency to be increased while still maintaining high efficiency. The higher switching frequency allows smaller magnetic circuit elements and capacitors to be used achieving cost savings and an increase in power density. One limitation is that the higher switching frequency also brings higher electromagnetic interference, particularly in circuit topologies that have pulsating input or output currents. This problem could be alleviated if the zero voltage switching cell could also provide an inherent filtering action.
Another shortcoming of some zero voltage switching schemes is that an additional active switch is required and often the second switch requires a high side driver which can be accomplished with an IC made specifically for high side drive or by using a gate drive transformer. An example of such a circuit is illustrated in FIG. 12 of U.S. Pat. No. 5,402,329. The gate drive transformer or high side driver circuit adds expense and space to the power converter. A zero voltage switching cell that does not require a high side drive mechanism would provide a unique advantage.
An object of the subject invention is to provide a power converter which is relatively simple and is capable of delivering output power at high efficiencies and high switching frequencies.
Another object of the subject invention is to provide a generally applicable zero voltage switching cell that when substituted for the main switch of a hard switching power converter eliminates first order switching losses and provides non-pulsating input terminal currents.
Another object of the subject invention is to provide a generally applicable zero voltage switching cell that when substituted for the main switch of a hard switching power converter eliminates first order switching losses and does not require a high side drive mechanism.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.
These and other objects of the invention are provided by a novel circuit technique that uses a generalized active reset switching cell consisting of two switches, a reset capacitor, and a small resonator choke or coupled inductor. The critical zero voltage switching transitions are accomplished using the stored magnetic energy in the small resonator choke or coupled inductor.